1. Field of the Invention:
The invention relates to methods and circuits for controlling stepping motors, and particularly to low power methods and circuits for controlling stepping motors.
2. Description of the Prior Art:
Stepping motors are widely utilized in many kinds of precision mechanical equipment requiring precise, controlled positioning. Stepping motors are capable of rapidly incrementally rotating their rotors by small precise angles. For example, the 6000 series stepping motors manufactured by Superior Electric Company can increment their rotors by "step" angles of 1.8.degree..+-.5% when driven in their full step mode and 0.9 when driven in their half-step mode. A stepping motor is controlled by digital current signals which cause the stator windings of the stepping motor to be energized in a predetermined pattern. A wide variety of stepping motor control systems for producing the necessary digital control signals are commercially available.
In many applications, it is highly desirable that a stepping motor be capable of being "stepped" or incremented at a very high rate of speed, even when the stepping motor is driving a full load. It is also frequently desirable that the stepping motor be able to or maintain a particular step position or angle and strongly resist any deviation from that step position or angle until subsequent stepping signals are received from the stepping motor control system. Unfortunately, in order to achieve high stepping rates and or high holding torques, very large amounts of power must be supplied to the starter windings of the stepping motor.
Accordingly, it is an object of the invention to provide a stepping motor driver system which achieves high stepping rates, yet requires substantially less power than known stepping motor driver systems.
Another difficulty associated with conventional stepping motor systems is that when known control systems are designed to attain high stepping rates, very large amounts of torque are applied to the rotor to increment it from one step position to the next. This is especially true when a large amount of mass including the mass of the rotor and the mass of a load driven by the rotor, must be accelerated and stopped during a stepping operation. The large amounts of torque frequently result in considerable "overshoot" of the rotor, causing undesired oscillation of the rotor and mass attached thereto about the desired step position or angle. The oscillation continues until it is damped out by whatever damping friction, such as bearing friction, may be available. The time required for damping of such mechanical oscillations is frequently a serious limiting factor in operation of the system driven by the stepper motors.
Accordingly, it is another object of the invention to provide a stepping motor system including stepping motor driver circuitry which achieves high stepping rates, yet reduces the amount of rotor overshoot associated with prior stepping motor control systems and also reduces the amount of time required for damping of oscillations of the rotor about a desired step position.
When stepping motor rotors are advanced to a particular step, angle or position, they are "held" at that step position by a relatively high holding torque. A conventional technique for controlling current applied to the stator windings to achieve rapid current buildup in the windings and also to attain a predetermined average winding current in order to attain high stepping rates and high holding torques involves repetitively switching of terminals of the stator windings to a high voltage source. This results in a high rate of current increase or build-up selected rotor windings. Conventional stepping motor drive systems frequently switch the stator winding currents to the high voltage source at rates in the range from three to five kilohertz. The resulting collapsing and expanding magnetic fields in the stepping motor stator windings interact with metallic parts of the stepping motor, causing them to produce an irritating hum or buzz.
Accordingly, still another object of the invention is to provide a stepping motor driver system which reduces or eliminates audio hum.